#include <iostream>
#include <stdio.h>

using namespace std;

int main() {
	int x,a,y,b;
	double s;
	cin >> x >> a >> y >> b;
	s = 1.0*(y * b - x * a)/(b-a);
	printf("%.2lf",s);
	//d:int f:double s:string

	return 0;
}